The present invention generally relates to communications systems and, more particularly, to a receiver.
In the ATSC (Advanced Television Systems Committee) standard for digital terrestrial television (DTV) in the United States (e.g., see, United States Advanced Television Systems Committee, “ATSC Digital Television Standard”, Document A/53, Sep. 16, 1995), the modulation system consists of a suppressed carrier vestigial sideband (VSB) modulation with an added small in-phase pilot at the suppressed carrier frequency, 11.3 dB below the average signal power, at the lower VSB signal edge. An illustrative frequency spectrum for an ATSC VSB signal is shown in FIG. 1.
In most communications systems, such as ATSC, the receiver uses “blind” algorithms to perform carrier and timing synchronization with the transmitted waveform, where the algorithms do not use any information about the transmitted symbols. For example, an ATSC receiver utilizes the above-noted small in-phase pilot tone to achieve carrier frequency lock, after which some other blind method, such as the well-known Gardner's algorithm, is used to achieve symbol-timing lock. However, when methods such as Gardner's algorithm are used with an ATSC VSB signal, they are sensitive to the carrier lock, such that if the carrier frequency lock is disturbed (e.g., when channel reflections distort pilot magnitude and/or phase), the symbol timing recovery (STR) loop will also be adversely affected. The opposite may also be true, when degradation in the STR performance leads to a corresponding degradation in the carrier tracking loop (CTL) performance. In addition, with the two functions being inter-related, the receiver's acquisition of a new transponder channel may take longer than it otherwise would.
Besides the above-described pilot-based CTL method and Gardner-based STR method, other methods, such as the well-known Costas loop, may be used to achieve the same goal: a squaring loop acting on the lower (pilot) edge of the received near-baseband VSB signal would produce a pseudo-tone (PST) whose frequency location indicates carrier offset, while the squaring loop acting on the opposite edge of the VSB signal would produce a pseudo-tone whose location relative to the pilot (or pseudo-pilot) indicates the symbol-timing offset. For this scheme to work, the lower PST is being driven to DC by the CTL, while the upper PST is being driven to 5.38 MHz (in ATSC) away from DC by the STR. However, as the loops converge, they may chase one after another, thus producing unwanted effects, such as delay in acquisition and instability. In addition, since the PST for the symbol timing offset is still generated relative to the location of the pilot, symbol timing lock is still adversely affected if the pilot is distorted in magnitude and/or phase.